Numbers 8:6-11, 21

“Take the Levites from among the children of Israel and cleanse them ceremonially. Thus you shall do to them to cleanse them: Sprinkle water of purification on them, and let them shave all their body, and let them wash their clothes, and so make themselves clean. Then let them take a young bull with its grain offering of fine flour mixed with oil, and you shall take another young bull as a sin offering. And you shall bring the Levites before the tabernacle of meeting, and you shall gather together the whole congregation of the children of Israel. So you shall bring the Levites before the LORD, and the children of Israel shall lay their hands on the Levites; and Aaron shall offer the Levites before the LORD like a wave offering from the children of Israel, that they may perform the work of the LORD.

And the Levites purified themselves and washed their clothes; then Aaron presented them like a wave offering before the LORD, and Aaron made atonement for them to cleanse them.